The present invention is directed to acoustic wave devices and, more particularly, to surface acoustic wave filters.
In the prior art, a number of acoustic wave devices are based on the interaction between an acoustic wave traveling on the surface of the crystal and a surface perturbation, such as a shallow groove grating on that surface. Such devices include filter banks, resonators, and reflective array compressors (RAC's). These prior art devices have all been "single surface" devices, that is, the input surface acoustic wave, the surface perturbation, and the output surface acoustic wave are all on the same surface of the crystal.
By way of example, a prior art filter may be constructed on the top surface of a crystal by placing a shallow groove grating on that surface. In response to a broadband surface excitation, a spectrum of surface acoustic waves (SAW's) may be generated which travel toward the grating at an oblique angle. At the grating, the input SAW's are split into two components. For a component of the input SAW where the inter-groove spacing (in the propagation direction of the incident SAW) of the grating is equal to the SAW wavelength, a first output SAW component is produced which travels in a direction having its angle of reflection with respect to the grating grooves equal to the angle of incidence with respect to those grooves. The other portion of the input SAW produces a second output SAW component which travels from the grating along the crystal surface in substantially the same direction as the incident SAW. The reflection coefficient for the first output component is proportional to the ratio of the depth of the groove to the wavelength of the incident wave. With this configuration, a sensor which is aligned to detect the first output SAW component exhibits a transmission pass-band at the frequency associated with the grating spacing. Similarly, a sensor which is aligned to detect the second output SAW component exhibits a transmission stop-band at the frequency associated with the grating spacing. While this prior art configuration does provide stop and pass-band filters for a surface acoustic wave, the Q of the band characteristic is relatively low.
Another form of prior art device includes a pair of parallel shallow groove gratings on a single surface of a crystal, with the elements of each grating having a separation equal to one-half the wavelength of the characteristic frequency for the resonant peak. The device provides a resonator peak at the characteristic frequency. However, this configuration is relatively limited with respect to its Q characteristic.
The prior art reflective array compressor devices generally include two shallow groove gratings on a single crystal surface. The elements of both gratings have matching monotonic inter-groove separation functions. The two gratings are set at an angle to each other so that a broad band SAW directed at the first grating travels toward the first grating, and the various frequency components of that SAW are reflected on the grooves having a spacing matching the wavelength of the components. The reflected components travel to the correspondingly spaced grooves of the second grating, and are then in turn reflected to an output transducer. By conventional techniques, the two gratings may be adapted to provide a phase response corresponding to a linear chirp, i.e., the phase response is a quadratic function of frequency. These prior art devices have been found to have relatively high loss due to the SAW reflection coefficients.
While the above described examples from the prior art are defined to make use of shallow groove gratings, it is known that other forms of surface perturbations may be used, such as films, or field shorting elements for piezoelectric crystals.
It is an object of the present invention to provide improved acoustic wave devices.
Another object is to provide a relatively high Q acoustic wave filter.
It is still another object to provide a new and improved acoustic wave reflective array compressor device.
Yet another object is to provide a relatively high finesse acoustic wave resonator.